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Freezing temperatures can cause lethal injuries in living plant tissues (Sakai & Larcher, 1987; Wisniewski et al., 1991; Guy, 2003; Kuroda et al., 2003) and are a major factor limiting the long distance transport of water in the xylem (Améglio et al., 2002; Cochard & Tyree, 1990; Améglio & Cruiziat, 1992; Sperry et al., 1994; Sperry, 1995; Améglio et al., 2001a; Feild & Brodribb, 2001; Cavender-Bares, 2005). The ability of different species to avoid or tolerate freezing stress through various mechanisms can go a long way in explaining their geographic distributions (Burke et al., 1976; Parker, 1963; Sakai, 1970; George et al., 1974; Sakai et al., 1981). Both drought and freezing are hypothesized to be important stress factors affecting the ecology and evolution of plants in the Mediterranean region (Damesin & Rambal, 1995; Mitrakos, 1980; Lo Gullo & Salleo, 1993; Damesin et al., 1998; Garcia-Plazaola et al., 1999), although the adaptation of plants to drought has traditionally been a larger research focus (Mooney & Dunn, 1970). Freezing stress, however, is more likely to limit the ranges of Mediterranean plants, including the numerous oak species of this region (Larcher, 1981; Tognetti et al., 1998; Nardini et al., 2000; Quézel & Médail, 2003). In this study, we compare the freezing sensitivity of four Mediterranean oak species growing in a garden in southern France, in terms of xylem embolism and photosynthetic function of leaves. These species vary in leaf lifespan (Table 1) and latitudinal distribution (Fig. 1).
Table 1. Leaf lifespans (days) and Northern range limits (degrees N, latitude) of four Mediterranean oak (Quercus) species
| ||Leaf lifespan||N range limit|
Figure 1. Distribution maps of each of the four Quercus species examined in the common garden study. Maps are redrawn from Quézel & Médail (2003). In (b) distributions for both Q. faginea (light gray) and Q. canariensis (dark gray) are shown, as these are sometimes considered a single species.
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Winter nights in the Mediterranean region of southern Europe and North Africa frequently reach freezing temperatures during winter months (December, January and February) but only rarely extend below −10°C. Both daily water transport through the xylem and leaf photochemistry are known to be impaired by night-time freezing temperatures as a result of freeze-thaw embolism and impairment of enzymatic processes involved in photosynthesis.
Xylem embolism formation in response to freezing stress has been shown to be dependent on vessel diameter (Davis et al., 1999; LoGullo & Salleo, 1993; Sperry, 1995; Pittermann & Sperry, 2003). The degree of xylem embolism is also sensitive to the minimum temperature experienced during freezing (Pockman & Sperry, 1997), although the effect of decreasing temperature may only be pronounced in some species (Pitterman & Sperry, 2003). Currently, it is unknown whether these contrasting patterns correspond to leaf phenology, although it has long been observed that hydraulic architecture and leaf phenology are coordinated, and it has been hypothesized that they evolve in concert (Lechowicz, 1984). In a study of hydraulic properties and freezing-induced xylem embolism in 17 oak species in north central Florida, USA, Cavender-Bares & Holbrook (2001) found that evergreen species tended to have smaller maximum vessel diameters and diffuse porous anatomy relative to co-occurring deciduous species with large diameter early wood vessels and ring porous anatomy. Loss of xylem conductivity after freezing was correlated with leaf phenology indicating that hydraulic architecture and phenology may be coevolved traits in oaks. Leaves of deciduous species have been shown to possess lower tolerance to chilling and freezing stress compared with evergreen species (Sakai et al., 1981; Adams et al., 1995; Cavender-Bares et al., 1999). Acclimatization to cold temperatures in overwintering evergreen species, including in oaks, has been linked to increases in antioxidants and xanthophyll pigments (Adams et al., 1995; Garcia-Plazaola et al., 1997; Garcia-Plazaola et al., 1999), as well as changes in the composition of photosystem II antenna and increases in cyclic electron transport that allow increased quenching of absorbed light (Öquist & Huner, 2003). Deciduous leaves are programmed to senesce and abscise in response to cold temperatures, potentially obviating the need for mechanisms to protect the photosynthetic apparatus during freezing. Evergreen leaves, on the other hand, even of species occurring in relatively warm climates such as northern Florida or Mediterranean Europe, would be expected to benefit in terms of carbon gain by maintaining function under mild freezing stress. Maintenance of leaf function should be greatest in the species with the longest leaf lifespan. However, maintaining functional leaves without functional xylem should be of little benefit. It is likely therefore that losses of function in the leaves and xylem are coordinated.
We hypothesized that the degree of sensitivity of the xylem to freezing temperatures should be related to leaf habit (evergreen and deciduous) or leaf lifespan and coupled to the sensitivity of the photosynthetic apparatus to freezing. We took advantage of a previously established common garden in Montpellier, France (Acherar & Rambal, 1992) where young trees of four Mediterranean oak species, two evergreen (Quercus ilex and Quercus suber) and two deciduous (Quercus afares and Quercus faginea), had been growing under the same conditions for 17 yr. Current ranges of each species are shown in Fig. 1 (based on Quézel & Médail, 2003). We examined the responses of leaves and stems of whole shoots to experimental overnight freezing and asked the following questions: (1) Is leaf longevity correlated with hydraulic traits and freezing sensitivity of leaves and xylem among closely related species? (2) Does loss of function in both xylem and photosynthesis of leaves increase with declining minimum temperatures during freezing? (3) Is there evidence for a cold acclimatization response in the xylem as well as in the photosynthetic apparatus, and does it differ between evergreen and deciduous species?